Mg-doped Na3V2-xMgx(PO4)2F3@C sodium ion cathodes with enhanced stability and rate capability

Abstract

To address the inherent low conductivity challenge of NVPF, carbon-coated, Mg2+-doped Na3V2(PO4)2F3 (denoted as Na3V2-xMgx (PO4)2F3@C) have been synthesized via a facile sol-gel method. The samples are denoted as NVPF, NVPF-Mg01 (x = 0.01), NVPF-Mg03 (x = 0.03), NVPF-Mg05 (x = 0.05), NVPF-Mg07 (x = 0.07), and NVPF-Mg10 (x = 0.10). A comprehensive study of Mg2+ doping and its effects on the crystal structure and electrochemical features of NVPF is carried out using a combination of XRD, XPS, SEM, TEM, GCD, CV, and EIS methodologies. The electrochemical performance of Na3V1.95Mg0.05(PO4)2F3@C (NVPF–Mg05) surpass others, with a specific capacity of 126.8 mAh g−1 at 0.1C (1C = 128 mAh g−1) and an impressive 96.72 % capacity retention over 100 cycles. At 10C, the initial discharge capacity is quantified at 102.3 mAh g−1, and the capacity retention reached 70% after 1000 cycles, surpassing 2000 cycles in overall cycling life. The discharge capacity at 0.1C is in close proximity to the theoretical specific capacity of NVPF, whereas at 10C, it experiences a notable increase of around 125.70% compared to undoped NVPF. The incorporation of Mg2+ through doping enhances the structural stability of NVPF, diminishes charge transfer impedance, and ameliorates the kinetics of NVPF.